Some spiral galaxies larger than previously reported!

Telescopes are picking up fainter and fainter stars on their edges. As a result, it turns out that many of these galaxies may be a good bit bigger than astronomers had thought -- including our own Milky Way.

One larger galaxy is NGC 300.

Astronomers had pegged its diameter at about 50,000 light-years -- about half the size of the Milky Way. But a few months ago, an international team of astronomers reported that the galaxy is actually twice that size. This extended disk contains lots of faint stars. The number of stars just keeps thinning out as you go farther and farther from the galaxy's core. The team used a giant new telescope in Chile to see the fainter stars.

A few months earlier, another team had reported that the Andromeda galaxy is about three times bigger than previously thought.

The findings suggest that the Milky Way may be bigger than expected, too. It's hard to determine, though, since we're inside the galaxy, so we can't see the whole disk. We have to look through swarms of nearby stars, plus dark clouds of gas, to see its edge.

I think you'd be surprised by how many seemingly "basic" facts elude astronomers because of observational limitations. Some of the most notable examples of this involve objects of low surface brightness, such as low-mass dwarf galaxies or the outer regions of large galaxies. It's entirely possible (likely, even) that there exist galaxies in our local group that have not been observed simply because their light is too diffuse.

I'm curious, though, how they're defining the "diameter" of these galaxies. It's not enough to simply find stars, since there exist stars even between galaxies. Perhaps the best measure would be the distance to which there exist "bound" stars, but I would it hard to verify that they were bound with such a low surface brightness.

I would like to learn more. I am not surprised by the limitations from 20-30 years ago. With Hubble, Keck and others, and modern electronic imaging, we certainly have learned more.

I found this -

31st May 2005
Andromeda Galaxy is 3 times the Expected Size

The group of scientists was represented by Scott Chapman, a researcher from the California Institute of Technology. He presented the results of a survey of Andromeda’s stellar motions at a meeting of the American Astronomical Society.

They used the Keck telescope to measure speeds of 5,000 stars in the outskirts of Andromeda to discover these latest facts. Scott expressed surprise at the fact that these fringe stars are rotating as if they are part of the galaxy’s nebula. The statement said: “Finding all these stars in an orderly rotation was the last explanation anyone would think of.”

The latest findings also claim that nebula is 220,000 light-years in diameter, instead of the earlier estimate of 70,000 to 80,000 light-years.

Ok, that web page clears things up a bit. From an astronomer's point of view, it would be a little weird to say that the galaxy was found to be three times bigger. We would break a spiral galaxy down into components:

- Dark Matter Halo: Contains most of the mass, extends to an unknown distance.
- Disk: Contains stars and gas (usually), often multi-component (thin and thick disk), emits most of the light. Has a sharp cutoff.
- Spheroid/bulge: Mostly old stars concentrated near the center of the galaxy. The smallest component.
- Stellar Halo: Small portion of the mass, but extends out to an unknown distance and emits very little light.

What he emphasizes on his website is that they've discovered an extended stellar disk. This is more interesting to an astronomer's ears because of the general picture outlined above. Stellar disks are thought to truncate at a specific radius and his discovery implies one or more of a number of things:

1) The above picture is oversimplified and there is a more extended component to most disks that we're unable to detect.
2) Something is or has happened to M31 in the recent past and this is what remains.
3) This is a separate structure with a similar angular momentum vector.
4) M31 or the observation are a fluke and there's nothing interesting to be taken from this.

I doubt it's the last option, but we should never rule it out.

Note that he also sees stars belonging to the stellar halo (at the same distance from M31's center) in his observations, but doesn't make a big deal of it. This is because we expect them to extend out to large radii and become unobservable. One can certainly model the stellar halo with a some kind of scale length fitting parameter, but I think few astronomers would think of it as the galaxy's size.

Anyway, good find, Astronuc. Sorry to be verbose, but I thought the news article wasn't doing it justice.

This would make up a very tiny fraction of the total "dark" matter. In fact, since it lies at such large radii, it wouldn't even make a contribution to the flattening of previously observed rotation curves.

Thanks Space Tiger - your response is not verbose at all, but rather for what I was hoping and expecting. I agree that the news blurbs are way too brief and simplistic, i.e. for general consumption. I was hoping that others, like yourself, could elaborate on this news and the implications. Many thanks!

I still have to wonder why the bright stars in one region and the darker/colder (older ?) stars in the outer band.

Anyhow, tripling the diameter of spiral galaxies seems pretty significant - the area now expands by a factor of 9, but perhaps the density is similar (or less, or greater?). So the mass would be several times greater!

And supposedly, the Milky Way is also larger!

It will take some time to digest and contemplate this matter.

As you mentioned, M31 could be an anomaly. However, NGC300 seems to similar larger than previously observed. Here is an article on NGC300 -http://www.physorg.com/news5730.html
The research is publishedin the Astrophysical Journal 10 August 2005.

Using the Gemini Multi-Object Spectrograph instrument on the Gemini South telescope in Chile, the observers were able to see stars in the disk up to 47,000 light-years [14.4 kpc] from the galaxy’s centre—double the previously known radius of the disk.

The finding has profound implications for our own Galaxy. Most current estimates put its size at 100,000 light-years across, about the same as the new estimate for NGC 300. “However, our galaxy is much more massive and brighter than NGC 300. So on this basis, our Galaxy is also probably much larger than we previously thought—perhaps as much as 200,000 light-years across,” said the paper’s lead author, Professor Joss Bland-Hawthorn of the Anglo-Australian Observatory.

“We now realize that there are distinctly different types of galaxy disks,” said team member Professor Ken Freeman of the Research School of Astronomy and Astrophysics at the Australian National University. “Probably most truncate—the density of stars in the disk drops off sharply. But NGC 300 just seems to go on forever. The density of stars in the disk falls off very smoothly and gradually.”

The observers traced NGC 300’s disk out to the point where the surface density of stars was equivalent to a one-thousandth of a Sun per square light-year.

“This is the most extended and diffuse population of stars ever seen,” said Bland-Hawthorn.

So NGC is twice the diameter as previously thought. So what about other spirals?

I still have to wonder why the bright stars in one region and the darker/colder (older ?) stars in the outer band.

I'm not sure what you mean here. Can you explain a bit more?

Anyhow, tripling the diameter of spiral galaxies seems pretty significant - the area now expands by a factor of 9, but perhaps the density is similar (or less, or greater?).

You could say that the surface area of the disk has been increased by that factor, but the newly discovered parts are extremely low density as compared to the inner parts. Imagine a frisbee with a ring of dust around it.

In it, he says that the new portions of the disk make up only 10% of the luminosity (despite being most of the area). A similar number would apply for the total stellar mass.

So the mass would be several times greater!

This doesn't really change our estimate of the total mass of the galaxy. Those estimates are usually obtained dynamically, which means that we observe the motions of stars, dust, etc. in the vicinity of Andromeda and infer the strength of the gravitational field required to maintain that motion. We then use Newtonian gravity to infer a mass from the gravitational field. Such measurements have already been made at radii greater than this extended disk, so its mass is already included in the estimated total mass of the galaxy.

As you mentioned, M31 could be an anomaly. However, NGC300 seems to similar larger than previously observed.

In the paper on M31, they were claiming that the "extended disk" was distinct from the rest of the Andromeda disk and represented the possible need to include more disk components when modeling spiral galaxies. In this case, they appear to be observing that the disk is all the same component and its luminosity profile is a smooth exponential out to very large radii.

So NGC is twice the diameter as previously thought. So what about other spirals?

NGC 300 is, in fact, somewhat odd because the majority of spirals appear to show a "break" at a few scale lengths in the exponential profile. As the paper notes, NGC 300's disk seems uninterrupted out to 10 scale lengths. It's very likely that other spirals will have disk material out to radii that we cannot currently observe, but I think the majority of them would have to be more like M31 (that is, with an extra component).

I still have to wonder why the bright stars in one region and the darker/colder (older ?) stars in the outer band.

ST said:

I'm not sure what you mean here. Can you explain a bit more?

I was just wondering what is different between the stars in the fainter regions and those in the brighter regions. Are the stars in fainter/darker region generally smaller, e.g. mostly dwarfs (or smaller stars)?

[I seem to remember reading a comment that most of the visible stars (I believe by apparent magnitude) are visible because they are giants or supergiants, and that probably refers to stars in the Milky Way.]

Or is this region dim because the stellar density is so low? Or is there more in the way of gas clouds in this region?

Bear in mind, I have been away from in-depth study of cosmology and astrophysics for more than two decades, so I lack good resources at the moment in terms of the latest theories and models. I hope to rectify that difficiency in the near future. On the other hand, I have a heck of a lot of other things demanding my attention as well.

As for the mass of galaxies, what about the moment of inertia. Would that have made sense if astronomers did not see that fainter mass? Or is the faint area a component of the 'dark matter' controversy.

I recently heard somewhere a discussion that the velocities of stars toward the outer regions seemed to fast, and that is why models needed dark matter, and maybe astronomers have been 'seeing' only about 1/6th of the mass/matter.

I was just wondering what is different between the stars in the fainter regions and those in the brighter regions. Are the stars in fainter/darker region generally smaller, e.g. mostly dwarfs (or smaller stars)?

Well, it depends. A region is usually dimmer because there are fewer stars, but it can also be because the population is older or has a different metal content. In this case, it's primarily because there are fewer stars.

[I seem to remember reading a comment that most of the visible stars (I believe by apparent magnitude) are visible because they are giants or supergiants, and that probably refers to stars in the Milky Way.]

It's true that the only individual stars that can be resolved in external galaxies are giants or supergiants. In the Milky Way, we can pretty much see stars of any luminosity (in the solar neighborhood, that is).

Or is there more in the way of gas clouds in this region?

They see gas out to about 30 kpc, it seems, but it's observed in the radio (HI 21 cm). It doesn't contribute to the optical luminosity.

As for the mass of galaxies, what about the moment of inertia. Would that have made sense if astronomers did not see that fainter mass?
Or is the faint area a component of the 'dark matter' controversy.

It's not important for the dark matter issue. These stars make up a tiny fraction of the total mass of the galaxy.

I recently heard somewhere a discussion that the velocities of stars toward the outer regions seemed to fast, and that is why models needed dark matter, and maybe astronomers have been 'seeing' only about 1/6th of the mass/matter.

That's correct. There is a lot of evidence for dark matter but some of the first came from the rotation curves of galaxies.